Graphene nanoribbons, which are narrow strips of single-layer graphene, are one of the most promising alternatives to silicon in MOSFETs. However, two-dimensional (2D) graphene is a semimetal with no inherent bandgap. Opening a bandgap in graphene in a practical way is one of the most important milestones for the future of nanoelectronics. One approach to do so involves narrowing large-area graphene to create nanoribbons (GNRs) of width <10 nm, which provides band gaps in the range of 1 eV.
The potential of GNRs remains unrealized because of synthetic limitations. Top-down subtractive patterning approaches have been demonstrated to produce GNRs from large area graphene, graphite, or carbon nanotubes. A variety of patterning techniques, including subtractive lithography, electron beam lithography, the use of nanowire etch masks, chemical etching of graphene, chemical vapor deposition, sonicating graphite or graphene, spatially resolved reduction of graphene oxide, and unzipping carbon nanotubes produce GNRs, but do not simultaneously control the width, edge structure, or pendant functionality of the ribbons. These methods all fail to control the atomic structure of the edges of the GNRs, particularly in ribbons <10 nm wide. Methods to form GNRs by oxidatively “unzipping” single-wall or multiwall carbon nanotubes have been previously reported. However, solution-based oxidative unzipping strategies produce insulating graphene oxide ribbons that show inferior conductivity when reduced back to GNRs.
Bottom-up syntheses have produced impractically short ribbons thus far. Two approaches have been demonstrated to-date. Polymerized bis(anthracene) monomers that were sublimed onto crystalline metal surfaces into perfect GNR structures have been previously reported. This method requires formation of the GNRs on the metal surfaces. These GNRs are produced in minute quantities as insoluble submonolayers and are unlikely to be relevant for nanoelectronic devices. Bulk synthesis of GNRs from linear polymer precursors has also been reported. A linear polymer is prepared, which is oxidized to form the ribbon's remaining carbon-carbon bonds. The major disadvantage of this approach is that the polymers are quite sterically hindered and high molecular weight samples were not obtained. Thus, the ribbons are 2.7 nm wide (including alkyl groups) but are only an average of 9.2 nm long. The inability to obtain higher molecular weight polymers combined with the difficulty of elaborating the polymer structure severely limit the utility of this approach.